The present invention relates to a hydraulic master cylinder, in particular for controlling actuation of braking circuit of a motor vehicle.
A hydraulic master-cylinder has already been proposed having a housing defining a bore therewithin, inside which is disposed a piston which separates the bore into a supply chamber connected to a fluid reservoir, and a pressure chamber connected to a utilization circuit. The piston of this master cylinder has a radial shoulder which cooperates with an annular ring, generally of elastomeric material to establish a valve which prevents fluid communication between the supply chamber and the pressure chamber during actuation of the piston, while allowing limited communication during the return stroke of the piston when a further supply of liquid in the pressure chamber is required, resulting from a leakage made in the utilisation circuit, or from a bleeding operation of this circuit. During movement of the piston, closure of the communication between the supply and pressure chambers, is obtained by abutment of the radial shoulder against one of the lateral radial faces of the annular sealing ring, the peripheral surface of which is in sealing contact with the internal wall of the bore.
It has been noted that, in the event that the vehicle brakes are actuated in a jerky manner, for example during a partial brake pedal release followed by a rapid reapplication of said pedal, there results a substantial loss in the piston stroke. This loss in pedal stroke occurs during the return stroke of the piston because the annular ring may slightly disengage from the radial shoulder of the piston even though the pressure in the brakes is maintained at a relatively high level. The decrease or even loss of fluid-tightness between the pressure and supply chambers occurs frequently when the partial release of the brake pedal is rapid and when the friction forces between the peripheral surface of the ring and the inner wall of the bore are great.
In order to overcome this drawback, a first solution has been proposed in French Patent Specification No. 1.440.394 of which U.S. Pat. No. 3,300,973 corresponds thereto, wherein a leaf spring urges the ring against the radial shoulder so as to avoid premature opening of fluid communication between the reservoir and the pressure chamber. However, this solution has not proved to be satisfactory since the force of the leaf spring must be compatible with the following opposing requirements: firstly this force must have a value sufficiently great to counterbalance the forces caused by friction between the ring and the housing bore, and secondly, this force must have a value sufficiently low to allow a rapid fluid supply toward the pressure chamber in the event of fluid depletion in said pressure chamber.
It has also been remarked that, when impurities or contaminants are present within the fluid of the hydraulic circuit, fluid tightness between the elastomeric ring and the piston shoulder cannot always perfectly be insured and there results a fluid leakage from the pressure chamber toward the supply chamber with as a consequence, a reduction in the piston stroke.